A magnetoresistive head is a sensing or reading head which utilizes magnetoresistive elements to sense or read magnetic information inherent in a magnetic medium. A magnetoresistive element is one whose electrical resistance varies with varying incident magnetic fields.
Prior art magnetic heads have been used for a great many applications including coin sensing, currency and bill validation, and sensing other forms of scrip. Typically, inductive type magnetic heads have been used, which require a rapidly changing magnetic field in order for magnetic information to be detected. Conversely, magnetoresistive heads are capable of detecting and reading information inherent in a slowly moving or stationary magnetic field.
A standard technique for determining the spatial relationship of signals from a magnetic medium involves using a single magnetic sensor to sense the signals while recording the time between each magnetic event as the medium moves at some velocity in relation to the sensor. In such a single magnetic sensor system, sometimes noise events cause data reduction problems. In addition, difficulties arise regarding determining the time between each magnetic event because the separation between the magnetic events on the medium is calculated by multiplying the recorded times by the detected velocity. A problem in implementing this technique is finding an accurate and inexpensive means to determine the velocity of the medium with respect to the sensor. One option is to control the speed of movement of the medium to ensure consistent and even movement of the medium under the sensor. Although speed control does ameliorate some of the problems associated with velocity changes, it adds cost, size and complexity to the system. Further, speed control is not sufficient under certain conditions, for example, when the medium is controlled by a human such as when a bill is inserted into a validator or a credit card is swiped through a reader.
The velocity measurement typically consisted of either measuring the amount of time it took the medium to move over a fixed distance, or measuring the distance the medium traveled in a fixed period of time. Often the velocity measurement became available only after a region of interest passed under the sensor, and typically reflected a mere average of the velocities attained over the entire distance traveled. Alternatively, the velocity determination was made by sampling the movement of the medium at frequent intervals, utilizing a costly encoder apparatus or other means.
Another prior art technique for reading magnetic information required two tracks on the medium. The first track contained the magnetic information to be read, and the second track contained regularly occurring clocking data. This technique exhibited poor flexibility in accommodating variations in the spacing of the magnetic information on the medium. Poor resolution resulted when large step increments were chosen for a desired spatial measurement. Further, when small step increments were used, costly and complex means were employed to accommodate the resulting delicate nature of the reading apparatus. Yet further, this technique cannot be used in some applications such as currency validation because the structure of the test medium is fixed without a clocking track.
Applications where the accurate determination of magnetic spatial separation is important include banknote denomination and credit card magnetic strip reading. For example, the separation of the grid lines of magnetic ink on the portrait-side of a U.S. banknote varies slightly depending on denomination. The one-dollar, two-dollar and five-dollar banknotes have grid line separations of 200, 250 and 275 microns, respectively. In addition, other denomination banknotes contain these grid line separations alone and in combination, and contain other spacings. Prior art systems that detected the magnetic grid line separation of U.S. banknotes required costly apparatus of significant complexity. Furthermore, the prior art utilized complex techniques to extract magnetic data from credit card magnetic strips, because the velocity of a credit card can vary significantly over the length of the card when moved through a reader, especially if moved by hand.